What is better for fat loss when the primary goal is losing fat while maintaining muscle, should you be spending time lifting weights or doing aerobic or cardio activities.

Here are two studies which explain the research I discuss.

Maintaining Physical Performance: The Minimal Dose of Exercise Needed to Preserve Endurance and Strength Over Time
https://pubmed.ncbi.nlm.nih.gov/33629972

Effects of aerobic and/or resistance training on body mass and fat mass in overweight or obese adults
https://www.ncbi.nlm.nih.gov/pmc/arti...

Yes building muscle is valuable, but fat loss and losing fat mass is benefitted more from spending time doing cardio.